1. Field of the Invention
The present invention relates to a mobile communication system and a mobile communication method which are configured to allow a mobile station and a radio base station to execute communication using a specific frequency band.
2. Description of the Related Art
A major change in a mobile communication system, which is accompanied by advances in technology, is called a generation change. A system before such a change and a system after the change are recognized as different generations. To achieve the generation change smoothly, different frequency bands may be allocated depending on the generations. For example, in Japan, the 800 MHz band has been mainly allocated to the second generation mobile communication system, and the 2 GHz band has been mainly allocated to the third generation mobile communication system.
Here, allocations of the frequency bands are conducted internationally by the ITU-R (which stands for International Telecommunication Union—Radio Communication Sector), and domestically by administrative agencies in each country. The 2 GHz band has been allocated internationally to the third generation mobile communication system in order to enable global standardization of radio frequency (RF) units as well as global roaming among systems adopting various modes included in the third generation mobile communication system.
If the second generation mobile system or other systems such as fixed wireless transmission were no longer in use and the number of users of the third generation mobile communication system were increased instead, the frequency bands previously allocated to the disused systems would be reallocated to the third generation mobile communication system. In such a case, the mobile communication system will be able to utilize a plurality of frequency bands.
In Japan, two frequency bands of the 800 MHz band and the 1.5 GHz band have been allocated to the second generation mobile communication system. Although only the 800 MHz band was allocated to the second generation mobile communication system in the beginning, the 1.5 GHz band was additionally allocated later with the increase in the number of users.
Conventionally, in the mobile communication system capable of utilizing a plurality of frequency bands, an applicable frequency band is selected either at random or considering the size of area coverage, radio capacity, and the like. Alternatively, since a traveling speed of a mobile station has an influence on communication quality, the applicable frequency band is also selected considering the traveling speed of the mobile station (refer to Japanese Patent Laid-Open No. H5(1993)-259969, for example).